1. Field of the Invention
The present invention relates to a pulse width modulated power amplifier. More particularly, the present invention relates to a Class D power amplifier where the modulation of the pulse width is preferably double-sided.
2. The Prior Art
Class D amplifiers have significant advantages compared to other types of power amplifiers such as Class A or Class B, especially with respect to power consumption. The output devices of a Class D amplifier operate in switched mode and, in the steady state, are either fully on or fully off. The fully on device has low impedance and, therefore, the power dissipation within the device itself is low. The fully off device is not passing current and dissipation is therefore low in this device also. In contrast, Class A or Class B devices operate for a large proportion of time with a voltage drop across the output device while they are conducting current and therefore dissipate power during these times. As a result, Class A and B devices are less efficient and reduce the useful battery life in battery-powered equipment and/or create the need for heatsinks in high output power stages.
Examples of Class D amplifiers include the Harris Semiconductor H1P4080 Class D amplifier described in "Class D amplifiers provide high efficiency for audio systems" Jeffrey D. Sherman, EDN May 25, 1995, pp. 103-110, and U.S. Pat. No. 4,592,087 to Killion. Both the Harris and the Killion references describe devices that accept input in analog form and operate on the principle of summing a triangular waveform together with an analog signal and comparing the result with a fixed analog voltage. The result of the comparison is used to generate the pulse width modulated output.
In "Reduction of Spectral Distortion in Class D amplifiers by an Enhanced Pulse Width Modulation Sampling Process", P. H. Mellor, S. P. Leigh, B. M. G. Cheetham, IEE Proc., Part G, 138, No. 4, 441-448 non-uniform sampling schemes have been reported which improve output quality compared to uniform sampling. However, a naturally sampled system is very difficult to achieve in a digital system because continuous interpolation must be performed to provide the adjusted input sample at the precise time of the output transition. Further, in applications such as portable audio systems and hearing aids the added complexities of the interpolation calculations are not justified for the intended audio quality levels.
Another circuit having Class D operation is the Delta-Sigma Digital to Analog Converter, in which a multi-bit digital representation is converted into a highly oversampled single bit output. The output is a train of pulses of equal width that, after filtering, produces the analog voltage corresponding to the multi-bit digital input. Delta-Sigma DAC architectures are described in "Delta-Sigma Data Converters Theory, Design and Simulation" IEEE Press, S. R. Norsworthy, R. Schreier, G. Temes Chapter 10. Operation is Class D since the output waveform is a train of digital pulses and the output devices are operated in switching mode. The analog filter used to filter the output pulses can be quite simple due to the high oversampling rate. In the case of audio signals, filtering can be achieved by the inductance of the loudspeaker itself. The oversampled converter provides a high precision output with low precision analog components at the cost of significant increase in complexity of the digital circuits.
According to this approach, power consumption, while good, is increased due to losses arising from the high frequencies and the large number of transients. It will appreciated that in applications such as hearing aids low, low power consumption is a critical issue. For example, in hearing aids, the circuit typically operates from a single zinc-aid cell that has a nominal voltage output of 1.3 Volts, a capacity of about 70 mAhr and, together with microphone, loudspeaker and audio processing circuits, must operate for at least 100 hours on a single cell.
It is therefore an object of the present invention to provide a low power, digital-to-pulse width modulated power amplifier.